Surface heat exchanger for compressible fluid alternative volumetric machines

ABSTRACT

The present invention relates to a surface heat exchanger ( 100 ) to be inserted within a compressible fluid volumetric alternative machine ( 200 ), within which a fluid moves in a not permanent motion, said heat exchanger comprising a plurality of walls, side by side each other, so shaped to realize at least a first duct ( 22 ) for the passage of a fluid in a permanent motion, and at least a second duct ( 22′ ) for the passage of said fluid in a not permanent motion, said first and second ducts ( 22, 22′ ) being configured so as to make said two fluids exchanging heat.

The present invention concerns the technical field of compressible fluid alternative volumetric machines, such as, e.g., industrial air and gas compressors, compressors for chemical or petrochemical industry, expanders, and it makes specific reference to an innovative surface heat exchanger to be installed in an compressible fluid alternative volumetric machine so that maximum amount of heat is exchanged at the same time work exchange realised during thermodynamic transformation occurring within said volumetric machine.

Until today, traditional compressible fluid volumetric machines are not provided with an inner heat exchanger.

At present, it is well know difficulty of exchanging heat during a thermodynamic transformation in a compressible fluid volumetric machine, for example extracting heat during a compression transformation in an operative machine or to add heat during an expansion transformation in a motive machine.

Gas evolving in a traditional alternative volumetric machine, as in all the compressible fluid volumetric machines, has a remarkable temperature variation during thermodynamic transformation realised in said volumetric machine, if said thermodynamic transformation is an adiabatic transformation, i.e. gas does not exchange heat with outer environment.

Fixing a starting point for said adiabatic transformation, defined by value of state variables (p, V, T), in an operative machine it is necessary spending a set amount of work so that a set mass of fluid reaches a set pressure value, while in a motive machine it is possible obtaining a set amount of work starting from a set fluid mass with respect to a set expansion ratio.

According to the first principle of thermodynamic, it would be suitable realising an isothermal transformation, and thus maintaining temperature constantly equal to the starting point temperature, n order to reduce the work amount to be spent to compress the same gas mass at the same final pressure value, or to increase the work amount that can be obtained by expansion of the same gas mass with respect to the same expansion ratio value.

In both cases, limits for realising an isothermal transformation is due to difficulties of fluid evolving within volumetric machine to exchange heat with outside, i.e. with a thermal source able to absorb heat generated during a compression or adducing heat during an expansion.

Said difficulty is due, on one hand, to an unfavourable value of ratio between exchange surface and volume within which thermodynamic expansion or S/V ratio occurs, and on the other hand to a low value of global heat exchange coefficient between gas evolving within volumetric machine and outer fluid carrier.

In order to improve S/V ratio, efforts have been focused on how increasing exchange surface.

For example, in order to increase surface of wall licked by a refrigerating fluid in compressors, fins have been provided on said wall.

However, results obtained by said solution to increase exchange surface are scarce.

In a thermal exchange, inverse of thermal exchange global coefficient, defined as global resistance, is substantially given by addition of two terms (convective thermal resistance of fluids), each one being opposite of convective exchange coefficient of relevant fluid. Therefore, in volumetric machine, one of the two terms is reverse of convective coefficient of a fluid carrier, usually a fluid having a permanent motion, and other term is reverse of convective coefficient of a fluid evolving within volumetric machine and it has not therefore a permanent motion.

Thus, it is obtained that thermal exchange global coefficient driven by fluid convective coefficient with the lower value between the two fluid convective coefficients.

Since value of convective coefficient of fluid evolving within the alternative volumetric machine is lower than fluid carrier coefficient, heat exchange is controlled by convective coefficient of said evolving fluid.

A drawback is due to the fact that values of evolving fluid convective coefficient can be difficulty obtained.

In fact, amount of data available, collected in a dimensionless way for a high number of geometries, refers to permanent motion conditions of both flows, and thus permits an easy proportioning of heat exchangers wherein fluids passing through are both in a permanent motion.

A further disadvantage is due to the difficulty of realising efficient heat exchangers with small dimensions so that they can be easily applied within the alternative volumetric machine, so that heat exchange occurs simultaneously with thermodynamic transformation realised in said volumetric machine.

Object of the present invention is that of overcoming said disadvantages, by providing a surface heat exchanger which is compact, to be applied within a compressible fluid alternative volumetric machine, so that maximum amount of heat is exchanged between fluid having a not permanent motion, evolving within the alternative volumetric machine, by a first conduct assembly, provided within the heat exchanger, and a fluid carrier, having a permanent motion, passing through a second conduct assembly of the heat exchanger, simultaneously to the work exchange during thermodynamic transformation occurring within said alternative volumetric machine, thus remarkably improving performances of the volumetric machine, as far as work given or obtained per kg of fluid is concerned.

Particularly, object of the invention is that of reducing unitary compression work for each evolving fluid mass unit, i.e. work to be provided to a compressor, or that of increasing unitary expansion work that can be obtained during within an expander from evolving fluid mass unit.

These and results are obtained, according to the invention, providing, within the volumetric machine, between movable member of the latter and its outlet or discharge valve, application of a compact heat exchanger, so realised to force compressed fluid having a not permanent motion to pass through the relevant conduct assemblies, so as to increase exchange surface between evolving fluid having a not permanent motion and fluid carrier having a permanent motion, thus improving S/V ratio of volumetric machine.

It is therefore specific object of the present invention a surface heat exchanger to be inserted within a compressible fluid volumetric alternative machine, within which a fluid moves in a not permanent motion, said heat exchanger comprising a plurality of walls, side by side each other, so shaped to realize at least a first duct for the passage of a fluid in a permanent motion, and at least a second duct for the passage of said fluid in a not permanent motion, said first and second ducts being configured so as to make said two fluids exchanging heat.

Particularly, heat exchanger can be provided with a core having a cylindrical shape, said cylindrical shape having any straight, regular or irregular, section contained within the straight section of a cylinder of said alternative volumetric machine, and said at least a second duct for passage of said fluid in a not permanent motion is parallel to the longitudinal axis of said cylinder.

According to the invention, said plurality of walls can be comprised of at least a group of three walls, wherein at least the first and the second walls are respectively provided with at least a groove, provided on a lateral surface of each one of said walls; said walls being provided so that said at least one groove of said first wall realises with the lateral surface of said second wall, a duct for passage of the fluid in permanent motion, and in that said at least one groove of said second wall realises a duct for passage of the fluid in a not permanent motion, along with the lateral surface of said third wall.

Always according to the invention, said at least one groove of said first wall has an equal or different orientation with respect to said at least one groove of said second wall, which is adjacent with respect to said first groove.

Furthermore, according to the invention, said at least one duct for passage of the fluid in permanent motion and/or said at least one duct for passage of the fluid in a not permanent motion has a rectangular section. If the shortest side of the rectangular section of said ducts varies, with the same longest side and the same duct length, inner volume of each duct varies, without substantially varying the exchange surface as well as the hydraulic diameter, and the ratio between the exchange surface and the volume, which is the inverse of the hydraulic diameter.

Particularly, lower side of rectangular section of each one of said ducts for passage of said fluid in a not permanent motion can be included within the interval of 0.1 mm and 1 mm.

Still according to the invention, orientation each other of said at least one groove for passage of fluid in permanent motion and said at least one groove for passage of the fluid in a not permanent motion is such to realise a cross flow heat exchanger, e.g. a perpendicular flow exchanger, or a parallel flow heat exchanger, e.g. a co-current exchanger or a counter current exchanger.

Always according to the invention, said group of walls is made up of a single piece, or by different elements assembled together.

Said grooves will be obtained in any case by any suitable technology known to those skilled in the art.

Further, it is specific object of the present invention a compressible fluid alternative volumetric machine comprising inside the surface heat exchanger as defined in the above, provided between the Top Dead Centre of a movable member of said volumetric machine, e.g. a piston, and a discharge valve or a discharge valve system.

In a first alternative embodiment, an intake valve or an intake valve assembly is provided, in the compressible fluid volumetric machine, close to the Tope Dead Centre of the movable member, at the exit of an intake duct provided in heat exchanger.

In a second alternative embodiment, an intake valve or an intake valve assembly is provided, in the compressible fluid volumetric machine, so as to take fluid in not permanent motion from outside, by a duct bringing said fluid from outside directly within a cylinder of said volumetric machine.

In the embodiment described, heat exchanger according to the invention is applied within a compressor, but it can be advantageously applied within every kind of compressible fluid alternative volumetric machine, e.g. an industrial gas compressor, a chemical or petrol chemistry industry compressor, an air conditioning compressor or an expander.

Present invention will be now described, for illustrative, but not limitative purposes, according to a preferred embodiment, making particular reference to the enclosed drawings, wherein:

FIG. 1 is a longitudinal section view of a known volumetric machine, such as an alternative compressor;

FIG. 2 is a longitudinal section view of volumetric machine of FIG. 1, with a heat exchanger according to the invention within the same;

FIG. 3 schematically shows path of a fluid having a permanent motion within heat exchanger of compressor of FIG. 2;

FIG. 4 is a top view of surface heat exchanger according to the invention;

FIG. 5 is a lateral view of surface heat exchanger according to the invention; and

FIG. 6 is an exploded view of a particular of surface heat exchanger, relevant to two walls of heat exchanger, each one having a lateral surface provided with grooves for passage, respectively in first wall, of a fluid having a permanent motion, and in second wall of a fluid, having a not permanent fluid.

Making reference to the figures, in the embodiment described, it is provided a surface heat exchanger, generically indicated by reference number 100, configured to be installed within a compressible fluid alternative volumetric machine, so that an amount of heat is exchanged simultaneously with work exchange during thermodynamic transformation occurring within said alternative volumetric machine.

In the example described, said alternative volumetric machine is a compressor 200, and said heat exchanger 100 is positioned within said compressor 200, between stop of a piston 203 (Top Dead Center or TDC), movable within a cylinder 201, and outlet valve or discharge valve 207 provided in head 202 of said compressor.

Heat exchanger 100 has a metallic mass for heat exchange, also indicated as core, generically indicated by reference number 103, comprising two duct systems, through one of which a fluid having a not permanent motion passes, and through the other one a carrier fluid having a permanent motion passes, preferably a refrigerating fluid.

Core 103 has a cylindrical shape, having a straight section included within the straight section of cylinder 201 of compressor 200. Particularly, core 103 ducts, parallel to longitudinal axis of compressor 200 cylinder 201, i.e. placed according to piston 203 motion, through which fluid having a not permanent motion directly passes, said fluid evolving within compressor 200, while remaining core 103 ducts, which are preferably perpendicular to the other ducts, are run by carrier fluid having a permanent motion, which is first brought to said core and then away from the same, after the heat exchange, by collector means.

Advantageously, direct passage of fluid having a not permanent motion through core 103 ducts, perpendicular with respect to the cylinder axis 201, prevents need of providing collector means for said fluid within heat exchanger 100, thus reducing add of dead volume with respect to the existing volume of volumetric machine.

Core 103 of heat exchanger 100 provides a plurality of walls 1, 1′ (in the embodiment shown in FIG. 6 it is provided a pair of walls 1, 1′) side by side each other, each one providing, on its lateral walls, one or more grooves, respectively grooves 2, 2′, comprising channels for passage of fluid having a permanent motion and a fluid having a not permanent motion.

Walls 1, 1′ are provided so that surface of wall 1 on which grooves 2 are provided touches wall surface 1′ without grooves to create ducts for passage of fluid having a permanent motion (FIG. 5).

As already said, even if not shown in FIG. 6, it is well evident that surface of wall 1′ on which grooves 2′ are provided contacts surface of a further wall 1 without grooves to realize passage ducts 22′ for fluid having a not permanent motion (FIG. 4).

In the embodiment shown in FIGS. 4-6, grooves 2 of wall 1 are perpendicular to grooves 2′ of adjacent wall 1′. As already said, orientation each other of grooves 2, 2′ can be different from the perpendicular one.

Lateral surfaces of ducts 22, 22′ comprised by said grooves 2, 2′ are exchange surfaces through which fluid having a not permanent motion yields heat to fluid having a permanent motion.

It is well evident that in case volumetric machine is an expander, through the same ducts, fluid having a not permanent motion receives heat from fluid having a permanent motion.

It is preferred that each one of ducts 22, 22′ has a straight section. This permits obtaining for each duct 22, 22′, that to a variation of lower side of straight section corresponds to a variation of S/V ratio, as well as of hydraulic diameter, proportional to inverse of said S/V ratio.

Particularly, decreasing of lower side of straight section, it is possible, with the same longer side and length of duct, varies inner volume of duct, without substantially varying exchange surface.

Longer side of straight section of ducts 22 for passage of fluid having a permanent motion and of ducts 22′ for passage of fluid having a not permanent motion can have any length, on the basis of core dimensions. Length of longer side of straight section of ducts 22 for passage of fluid having a not permanent motion can be different with respect to length of ducts 22′ for passage of fluid having a not permanent motion. With increase pressure of fluid having a not permanent motion, depending on use of type of alternative volumetric machine wherein it is introduced heat exchanger 100, it is necessary reducing length of longer side of straight section of each one of ducts 22 for passage of fluid having a not permanent motion, thus increasing number of ducts, so that they give a higher resistance to stresses due to inner pressure.

Lower side of straight section of ducts 22 for passage of fluid having a permanent motion and of ducts 22′ for passage of fluid having a not permanent motion can be the equal to lower than 1 mm.

Taking into consideration that walls 1, 1′, on which relevant grooves 2, 2′ are provided, can have a very thin thickness, it is understood that heat exchanger 100 can be characterized by a remarkable compactness with respect to traditional heat exchangers.

Size limit for section of said ducts is connected with needing of housing the highest number of ducts within heat exchanger core 103, to increase exchange surface, bearing in mind that inner volume of ducts 22′ for passage of fluid having a not permanent motion is as lower as possible, and that pressure drops within ducts 22 for passage of fluid having a permanent motion are very low.

Since pressure drops within ducts 22′ are not high, since relative speed of gaseous fluid having a not permanent motion within said ducts 22′ is low, it is possible moderately increasing length of cylinder comprising core 103 to keep carrier fluid speed, without increasing lower side of straight section of ducts 22 for passage of carrier fluid, and permitting at the same time reducing lower side of duct section 22′ for passage of gaseous fluid. Advantageously, inner volume of ducts 22′ is not increased and thus dead space added to the existing space of volumetric machine is the same.

Further, it is possible varying number of said grooves 2 varying number of grooved walls 1.

On the other hand, it is also possible varying lower side of straight section of each groove 2′ thus varying inner volume of each duct 22 for evolving fluid.

By reducing lower side of straight section of ducts 22′ for fluid having a not permanent motion, micro-turbulences of said fluid are caused within said ducts, particularly during the re-expansion stage of the fluid contained within dead space of compressor 200, obliging the same fluid to flow back, i.e. to invert its flow direction, even if for short time periods, with positive effects on convective exchange coefficient of fluid with a not permanent motion.

Reduction of section of ducts 22, 22′ made up of relevant grooves 2, 2′ can be done up to the permitted value useful to prevent pressure drop of fluid passing through.

In order to avoid that pressure drops cause lowering of pressure and that carrier fluid entering within heat exchanger exchanges heat with gaseous fluid with a permanent motion already compressed, during the intake phase of compressor, it is preferred that inlet valve or intake valve 205 of compressor 200 is displaced from a position close to the testate 202, to exit of an intake duct 102 provided in heat exchanger 100, still being close to the Top Dead Center, to permit to the entering fluid to fill in cylinder 201, without heating the latter fluid by heat exchanger walls (FIG. 2).

As an alternative, (not shown), intake valve 205 of alternative volumetric machine is so positioned to take gaseous fluid from outside, by a duct bringing it within cylinder 201 of compressor 200.

A first advantage of the solution according to the invention is due to the compactness of heat exchanger with respect to a known one. Compactness of heat exchanger according to the present invention is an important feature to permit its introduction within an alternative volumetric machine, without an increase of dead space of volumetric machine caused by inner volume of ducts for passage of gaseous fluid, and without reducing buckling of compression or expansion transformation, i.e. without reducing exponent of relevant polytrophic. The above means preventing that maximum pressure that can be reached within a compressor is reduced or increasing pressure during expansion phase within an expander. In other words, possibility of realizing a compact heat exchanger wherein increase of work volume due to inner volume of ducts through which fluid with a not permanent motion passes is limited with respect to increase of exchange surface. Thus, S/V ratio of volumetric machine provided with said heat exchanger is substantially higher than ratio of a traditional volumetric machine without said heat exchanger.

A second advantage is due to possibility of realizing, by choosing suitable sections of each duct for passage of fluids, a heat exchanger with dimension defined by cylinder bore of compressor on which heat exchanger is to be provided, maximum thermal exchange surface, minimum inner volume of passage ducts for gaseous fluid with not permanent motion and minimum pressure drops of passage ducts for carrier fluid with permanent motion.

A third advantage is that use of heat exchanger according to the invention is the high energy saving, in function of ratio between pressures and thus saves of costs. Said energy saving is given, for a compressor, as the lower work necessary to compress 1 Kg of fluid and for an expander, the higher work done during expansion of one Kg of fluid.

A fourth advantage is that fluid with a permanent motion within corresponding ducts characterized by values of relevant convective exchange coefficient that can be easily defined, thanks to huge amount of studies and test results available, along with the increase of convective exchange coefficient on side of fluid with a not permanent motion, permits an easy interpretation of global exchange coefficient.

A fifth advantage is due to the fact that heat exchanger can be used even where an amount of fluid evolves within a volume trapped between two shut off means to realize a transformation characterized by simultaneous exchanges of thermal energy and pressure, under periodical and varying kinetic conditions. As a consequence, heat exchanger can be also installed between two compressible fluid volumetric machines, regardless if they are of the same type, such as two compressors or two expanders, or different each other, such as a compressor and an expander. Particularly, heat exchanger can be positioned between two volumetric machines to realize a regenerative thermodynamic cycle. In this case, said two volumetric machines are so connected each other by two different ducts that fluid with a not permanent motion, evolving within first volumetric machine, enters within second volumetric machine, by a first duct, evolves within the second volumetric machine, going back to the first volumetric machine, through a second duct, wherein said first and second ducts are ducts of a heat exchanger that can be thus defined as regenerative.

A further advantage is due to the wide range of applications of the heat exchanger according to the invention.

In fact, said heat exchanger can be applied to any alternative volumetric machine, e.g. an alternative volumetric machine in an energetic system for production of mechanical work or in energetic system for air conditioning or in a system for obtaining temperatures under 0°, or even temperatures of about −180° C. or −250° C., transferring to the system the consequent energetic advantages.

The present invention has been described for illustrative, but not limitative purposes with reference to a preferred embodiment, but it well evident that one skilled in the art can introduce modifications to the same without departing from the relevant scope as defined in the enclosed claims. 

1. Surface heat exchanger (100) to be inserted within a compressible fluid volumetric alternative machine (200), within which a fluid moves in a not permanent motion, said heat exchanger comprising a plurality of walls, side by side each other, so shaped to realize at least a first duct (22) for the passage of a fluid in a permanent motion, and at least a second duct (22′) for the passage of said fluid in a not permanent motion, said first and second ducts (22, 22′) being configured so as to make said two fluids exchanging heat.
 2. Heat exchanger (100) according to claim 1, characterized in that it is provided with a core (103) having a cylindrical shape, said cylindrical shape having any straight, regular or irregular, section contained within the straight section of a cylinder (201) of said alternative volumetric machine (200), and said at least a second duct (22′) for passage of said fluid in a not permanent motion is parallel to the longitudinal axis of said cylinder (201).
 3. Heat exchanger (100) according to claim 1 or 2, characterized in that said plurality of walls can be comprised of at least a group of three walls (1 1′, 1), wherein at least the first and the second walls (1, 1′) are respectively provided with at least a groove (2, 2′), provided on a lateral surface of each one of said walls; said walls (1, 1′, 1) being provided so that said at least one groove (2) of said first wall (1) realizes with the lateral surface of said second wall (1′), a duct (22) for passage of the fluid in permanent motion, and in that said at least one groove (2′) of said second wall (1′) realizes a duct (22′) for passage of the fluid in a not permanent motion, along with the lateral surface of said third wall (1).
 4. Heat exchanger (100) according to claim 3, characterized in that said at least one groove (2) of said first wall (1) has an equal or different orientation with respect to said at least one groove (2′) of said second wall (1′), which is adjacent with respect to said first groove (2).
 5. Heat exchanger (100) according to claim 3 or 4, characterized in that said third wall (1) is provided with at least a groove (2) and in that said at least a groove (2) of said third wall has the same orientation with respect to said at least a groove (2) of said first wall (1).
 6. Heat exchanger (100) according to one pr claims 3-5, characterized in that said at least one duct (22) for passage of the fluid carrier in permanent motion and/or said at least one duct (22′) for passage of the fluid in a not permanent motion has a rectangular section; thus obtaining that if the shortest side of the rectangular section of said ducts (22, 22′) varies, with the same longest side and the same duct length, inner volume of each duct varies, without substantially varying the exchange surface; varying at the same time hydraulic diameter, which is equal to twice said shortest side, which is the ratio between the exchange surface and the volume, which is the inverse of the multiplication between said hydraulic diameter and the length of each duct.
 7. Heat exchanger (100) according to previous claim, characterized in that lower side of rectangular section of each one of said ducts (22′) for passage of said fluid in a not permanent motion is included within the interval of 0.1 mm and 1 mm.
 8. Heat exchanger (100) according to one pr claims 3-5, characterized in that orientation each other of said at least one groove (2) for passage of fluid in permanent motion and said at least one groove (2′) for passage of the fluid in a not permanent motion is such to realize a cross flow heat exchanger, e.g. a perpendicular flow exchanger, or a parallel flow heat exchanger, e.g. an co-current exchanger or a countercurrent exchanger.
 9. Heat exchanger (100) according to one of claims 3-8, characterized in that said group of walls (1, 1′, 1) is made up of a single piece, or by different elements assembled together.
 10. Heat exchanger (100) according to one of claims 3-9, characterized in that it comprises a plurality of groups of three walls (1, 1′, 1), side by side or adjacent each other.
 11. Heat exchanger according to anyone of claims 1-10, characterized in that said fluid in a not permanent motion is a gas and in that said fluid in permanent motion is a liquid.
 12. Compressible fluid alternative volumetric machine (200) comprising inside the surface heat exchanger (100) according to one of claims 1-11, provided between the Top Dead Centre of a movable member (203) of said volumetric machine, e.g. a piston, and a discharge valve (207) or a discharge valve system.
 13. Compressible fluid alternative volumetric machine (200) according to claim 12, characterized in that an intake valve (205) or an intake valve assembly is provided close to the Tope Dead Centre of the movable member (203), at the exit of an intake duct (202) provided in heat exchanger (100).
 14. Compressible fluid alternative volumetric machine (200) according to claim 12, characterized in that an intake valve (205) or an intake valve assembly is provided, in the compressible fluid volumetric machine, so as to take fluid in not permanent motion from outside, by a duct bringing said fluid from outside directly within a cylinder (201) of said volumetric machine. 